The field of the present invention is needleless hypodermic injection devices. More particularly, the invention relates to needleless hypodermic devices utilizing pressurized gas for injection of medication.
Various needleless hypodermic injection devices have been known in the past. For example, Morrow et al, U.S. Pat. No. 4,790,824 describes a needleless hypodermic injection device having a two-stage gas delivery system and an ampule shroud containing medication which is driven through the skin via gas pressure.
Parsons et al, U.S. Pat. No. 4,680,027 discloses an injection device using a pressurized gas cartridge to drive a piston against the biasing force of a spring. The driven piston works on a syringe causing liquid medication to be ejected with sufficient pressure to penetrate the skin of the patient.
While these and other known injection devices have met with varying degrees of success, their constructions or operating features can prevent effective injection. It has now been discovered that the injection of liquid medication should be as instantaneous as possible. With gas powered injection devices, the rise time of the gas pressure acting on the syringe, and the resulting acceleration at which the syringe is driven, is critical. When the gas pressure acting on the syringe rises too slowly, the initial medication ejected from the syringe does not have sufficient pressure or velocity to pass through the skin. In addition, if the "rise time" of the injection sequence is not sufficiently fast, a substantial portion of the medication will be too slowly driven from the syringe causing a "splash back" condition. Consequently, as a result of "splash back" the patient does not receive the full dosage of medication.
In gas driven injection devices, there are several factors which may affect the efficiency of the device. For example, devices having a long or tortuous gas path will have slower rise times due to flow losses and gas volume compressibility effects. In addition, certain injection devices rely on direct mechanical valve operation by the user of the device to release the gas pressure during the injection sequence. Since the valve operation is done manually in these devices, the effectiveness of the injection can vary widely with the user, due to the speed, activating force and completeness of activating movement employed by different users of the device. More importantly, it has not been previously appreciated that many of these types of devices have relatively large "dead" spaces or volumes of gas trapped behind the piston when the device in the ready to fire condition. These dead volumes substantially hinder injection by slowing the rise time of the gas pressure acting on the piston since substantial time is required for relatively large volumes of gas to flow into the dead volumes to build up an adequate injection pressure.
Accordingly, it is an object of the invention to provide an improved needleless hypodermic injection device.
It is another object of the invention to provide a novel ampule assembly which may be advantageously used with such a needleless injection device.
It is another object of the invention to provide a novel method of subcutaneous or intramuscular injection.